gpio: pca953x: Split pca953x_restore_context() and pca953x_save_context()

[ Upstream commit ec5bde62 ]

Split regcache handling to the respective helpers. It will allow to
have further refactoring with ease.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Stable-dep-of: 3e38f946 ("gpio: pca953x: fix IRQ storm on system wake up")
Signed-off-by: Sasha Levin <sashal@kernel.org>